U.S. patent number 11,322,735 [Application Number 16/791,222] was granted by the patent office on 2022-05-03 for lithium secondary battery.
This patent grant is currently assigned to SK INNOVATION CO., LTD.. The grantee listed for this patent is SK INNOVATION CO., LTD.. Invention is credited to Hee Gyoung Kang, Jong Hyuk Lee, Dock Young Yoon.
United States Patent |
11,322,735 |
Lee , et al. |
May 3, 2022 |
Lithium secondary battery
Abstract
Provided is a lithium secondary battery. The lithium secondary
battery includes a negative electrode including a negative
electrode active material layer, wherein the negative electrode
active material layer includes a mixed negative electrode active
material including graphite particles and low crystalline
carbon-based particles, and the negative electrode active material
layer has an apex of an exothermic peak in a temperature range of
no less than 370.degree. C. and no more than 390.degree. C., as
measured by differential scanning calorimetry (DSC).
Inventors: |
Lee; Jong Hyuk (Daejeon,
KR), Yoon; Dock Young (Daejeon, KR), Kang;
Hee Gyoung (Daejeon, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
SK INNOVATION CO., LTD. |
Seoul |
N/A |
KR |
|
|
Assignee: |
SK INNOVATION CO., LTD. (Seoul,
KR)
|
Family
ID: |
1000006282628 |
Appl.
No.: |
16/791,222 |
Filed: |
February 14, 2020 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200266422 A1 |
Aug 20, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Feb 15, 2019 [KR] |
|
|
10-2019-0018050 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/133 (20130101); H01M 4/583 (20130101); H01M
10/0525 (20130101); H01M 2004/027 (20130101) |
Current International
Class: |
H01M
4/133 (20100101); H01M 10/0525 (20100101); H01M
4/583 (20100101); H01M 4/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
10-0960138 |
|
May 2010 |
|
KR |
|
10-2014-0062125 |
|
May 2014 |
|
KR |
|
10-2017-0049080 |
|
May 2017 |
|
KR |
|
Primary Examiner: McConnell; Wyatt P
Attorney, Agent or Firm: IP & T Group LLP
Claims
What is claimed is:
1. A lithium secondary battery comprising: a negative electrode
comprising a negative electrode active material layer, wherein the
negative electrode active material layer comprises a mixed negative
electrode active material including graphite particles and low
crystalline carbon-based particles, and the negative electrode
active material layer has an apex of an exothermic peak in a
temperature range of no less than 370.degree. C. and no more than
390.degree. C., as measured by differential scanning calorimetry
(DSC), wherein the differential scanning calorimetry measurement is
performed by heating 6 mg of a specimen of the negative electrode
active material layer to 600.degree. C. at a rate of 5.degree.
C./min while feeding air to the specimen at a rate of 50 mL/min,
and wherein the low crystalline carbon-based particles have a Raman
spectrum R value (Id/Ig) of 0.9 or more.
2. The lithium secondary battery of claim 1, wherein the negative
electrode active material layer further has an apex of an
exothermic peak in a temperature range of no less than 450.degree.
C. and no more than 510.degree. C., as measured by differential
scanning calorimetry (DSC).
3. The lithium secondary battery of claim 1, wherein a heating
value of the exothermic peak having an apex at a temperature of no
less than 370.degree. C. and no more than 390.degree. C. is no less
than 50 J/g and no more than 200 J/g.
4. The lithium secondary battery of claim 2, wherein a heating
value of the exothermic peak having an apex at a temperature of no
less than 450.degree. C. and no more than 510.degree. C. is no less
than 20 J/g and no more than 60 J/g.
5. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles are capable of adsorbing and
releasing Li ions.
6. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles have a discharge capacity of 240
mAh/g or more for the second cycle when a negative electrode active
material is used as the low crystalline carbon-based particles and
a lithium metal is used as a counter electrode, and wherein the
discharge capacity is a discharge capacity determined under
charging/discharging conditions at which, after charging is
performed with a constant current at a rate of 0.1C, the charging
is stopped when a current flows at a rate of 0.01C in a constant
voltage mode until a constant voltage reaches 0.005 V, and
discharging to 1.5V is performed with a constant current at a rate
of 0.1C with a rest interval of 10 minutes.
7. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles are hard carbon particles.
8. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles have a D.sub.v50 of 4 .mu.m or
less and a D.sub.n50 of 1 .mu.m or less, and wherein the D.sub.v50
represents a particle diameter when a cumulative volume from a
small particle diameter accounts for 50% in measuring a particle
size distribution using a laser scattering method, and the
D.sub.n50 represents a particle diameter when a cumulative particle
number from a small particle diameter accounts for 50% in measuring
a particle size distribution using a laser scattering method.
9. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles have a D.sub.v10 of 2 .mu.m or
less and a D.sub.n10 of 0.5 .mu.m or less, and wherein the
D.sub.v10 represents a particle diameter when a cumulative volume
from a small particle diameter accounts for 10% in measuring a
particle size distribution using a laser scattering method, and the
D.sub.n10 represents a particle diameter when a cumulative particle
number from a small particle diameter accounts for 10% in measuring
a particle size distribution using a laser scattering method.
10. The lithium secondary battery of claim 1, wherein the low
crystalline carbon-based particles have a D.sub.v90 of 8 .mu.m or
less and a D.sub.n90 of 2.7 .mu.m or less, and wherein the
D.sub.v90 represents a particle diameter when a cumulative volume
from a small particle diameter accounts for 90% in measuring a
particle size distribution using a laser scattering method, and the
D.sub.n90 represents a particle diameter when a cumulative particle
number from a small particle diameter accounts for 90% in measuring
a particle size distribution using a laser scattering method.
11. The lithium secondary battery of claim 1, wherein the mixed
negative electrode active material comprises 5% by weight or less
of the low crystalline carbon-based particles, based on a total of
100% by weight of the mixed negative electrode active material.
12. The lithium secondary battery of claim 1, wherein the negative
electrode has a total pore area of 3.0 m.sup.2/g or less, as
measured by mercury intrusion porosimetry when the negative
electrode is pressed at an electrode density of 1.6 g/cc.
13. The lithium secondary battery of claim 1, wherein a rate of
increase (A) in the total pore area of the negative electrode is
less than or equal to 3.4: A=(V.sub.a-V.sub.b)/0.2 wherein V.sub.a
represents a total pore area (m.sup.2/g) of the negative electrode
measured at an electrode density of 1.7 g/cc by mercury intrusion
porosimetry, V.sub.b represents a total pore area (m.sup.2/g) of
the negative electrode measured at an electrode density of 1.5 g/cc
by mercury intrusion porosimetry, and 0.2 is calculated from the
equation: 1.7 g/cc-1.5 g/cc.
14. A lithium secondary battery comprising: a negative electrode
comprising a negative electrode active material layer, wherein the
negative electrode active material layer comprises a mixed negative
electrode active material including graphite particles and low
crystalline carbon-based particles, and in three or more points
having an area of 80 .mu.m.times.20 .mu.m optionally selected from
sections of the negative electrode active material layer, an area
of the low crystalline carbon-based particles accounts for no less
than 0.5% and no more than 8% with respect to the total area of the
sections, and wherein the low crystalline carbon-based particles
have a Raman spectrum R value (Id/Ig) of 0.9 or more.
15. The lithium secondary battery of claim 14, wherein, in the
three or more points having an area of 80 .mu.m.times.20 .mu.m
optionally selected from sections of the negative electrode active
material layer, the area of the low crystalline carbon-based
particles having a long diameter of 8 .mu.m or less accounts for no
less than 0.5% and no more than 8% with respect to the total area
of the sections.
16. The lithium secondary battery of claim 14, wherein, in the
three or more points having an area of 80 .mu.m.times.20 .mu.m
optionally selected from sections of the negative electrode active
material layer, a ratio of a diameter of the low crystalline
carbon-based particles to a diameter of the graphite particles is
less than or equal to 1/2, and wherein the diameter of the graphite
particles and the diameter of the low crystalline carbon-based
particles are the highest value selected from the long diameters of
the graphite particles and the highest value selected from the long
diameters of the low crystalline carbon-based particles,
respectively, in the optionally selected points having an area of
80 .mu.m.times.20 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. .sctn. 119 to
Korean Patent Application No. 10-2019-0018050, filed on Feb. 15,
2019, in the Korean Intellectual Property Office, the disclosure of
which is incorporated herein by reference in its entirety.
TECHNICAL FIELD
The following disclosure relates to a lithium secondary
battery.
BACKGROUND
In recent years, ardent research on high-capacity batteries has
been conducted to increase a driving range in order to
commercialize electric vehicles.
Because graphite commonly used as a negative electrode active
material for lithium secondary batteries has a low theoretical
capacity, it has limitations in increasing the driving range.
Therefore, active attempts have been made to apply a new
high-capacity negative electrode active material as a Si-based
negative electrode active material, and the like.
However, this research is still insufficient to commercialize the
electric vehicles, and requires a lot of time until the electric
vehicles are commercialized.
Therefore, as another plan, an approach of directing the
improvement of a charge/discharge rate instead of an increase in
the driving range may be considered in order to advance the
commercialization of the electric vehicles.
To improve the charge/discharge rate, lithium ions have to be
adsorbed and released to/from a negative electrode of a lithium
secondary battery at a high speed. In this case, graphite has a
problem in that it is difficult to realize high-current input
characteristics, which makes it difficult to rapidly charge and
discharge the lithium secondary battery.
Also, it is essential to prevent degradation of the battery under a
high-temperature environment and secure the long-term lifespan
performance in terms of the reliability of electric cars.
Accordingly, there is a need for the development of a new negative
electrode and lithium secondary battery having excellent battery
characteristics such as high-temperature storage efficiency and
lifespan characteristics, and the like while realizing the
high-current input characteristics so that the lithium secondary
battery can be rapidly charged and discharged.
SUMMARY
An embodiment of the present invention is directed to providing a
lithium secondary battery capable of realizing excellent
high-temperature storage efficiency and lifespan characteristics
while realizing high-current input characteristics so that the
lithium secondary battery can be rapidly charged and
discharged.
In one general aspect, a lithium secondary battery includes a
negative electrode including a negative electrode active material
layer, wherein the negative electrode active material layer
includes a mixed negative electrode active material including
graphite particles and low crystalline carbon-based particles, and
the negative electrode active material layer has an apex of an
exothermic peak in a temperature range of no less than 370.degree.
C. and no more than 390.degree. C., as measured by differential
scanning calorimetry (DSC).
The differential scanning calorimetry measurement is performed by
heating 6 mg of a specimen of the negative electrode active
material layer to 600.degree. C. at a rate of 5.degree. C./min
while feeding air to the specimen at a rate of 50 mL/min.
The low crystalline carbon-based particles may have a Raman
spectrum R value (I.sub.d/I.sub.g) of 0.9 or more.
The negative electrode active material layer may further have an
apex of an exothermic peak in a temperature range of no less than
450.degree. C. and no more than 510.degree. C., as measured by
differential scanning calorimetry (DSC).
A heating value of the exothermic peak having an apex at a
temperature of no less than 370.degree. C. and no more than
390.degree. C. may be no less than 50 J/g and no more than 200
J/g.
A heating value of the exothermic peak having an apex at a
temperature of no less than 450.degree. C. and no more than
510.degree. C. may be no less than 20 J/g and no more than 60
J/g.
The low crystalline carbon-based particles may be capable of
adsorbing and releasing Li ions.
The low crystalline carbon-based particles may have a discharge
capacity of 240 mAh/g or more for the second cycle when the
negative electrode active material is used as the low crystalline
carbon-based particles and a lithium metal is used as a counter
electrode.
The discharge capacity is a discharge capacity determined under
charging/discharging conditions at which, after charging is
performed with a constant current at a rate of 0.1C, the charging
is stopped when a current flows at a rate of 0.01C in a constant
voltage mode until a constant voltage reaches 0.005 V, and
discharging to 1.5V is performed with a constant current at a rate
of 0.1C with a rest interval of 10 minutes.
The low crystalline carbon-based particles may be hard carbon
particles.
The low crystalline carbon-based particles may have a D.sub.v50 of
4 .mu.m or less and a D.sub.n50 of 1 .mu.m or less.
The D.sub.v50 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 50% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n50 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 50% in
measuring a particle size distribution using a laser scattering
method.
The low crystalline carbon-based particles may have a D.sub.v10 of
2 m or less and a D.sub.n10 of 0.5 .mu.m or less.
The D.sub.v10 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 10% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n10 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 10% in
measuring a particle size distribution using a laser scattering
method.
The low crystalline carbon-based particles may have a D.sub.v90 of
8 .mu.m or less and a D.sub.n90 of 2.7 .mu.m or less.
The D.sub.v90 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 90% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n90 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 90% in
measuring a particle size distribution using a laser scattering
method.
The mixed negative electrode active material may include 5% by
weight or less of the low crystalline carbon-based particles, based
on a total of 100% by weight of the mixed negative electrode active
material.
The negative electrode may have a total pore area of 3.0 m.sup.2/g
or less, as measured by mercury intrusion porosimetry when the
negative electrode is pressed at an electrode density of 1.6
g/cc.
A rate of increase (A) in the total pore area of the negative
electrode may be less than or equal to 3.4.
A=(V.sub.a-V.sub.b)/0.2
wherein V.sub.a represents a total pore area (m.sup.2/g) of the
negative electrode measured at an electrode density of 1.7 g/cc by
mercury intrusion porosimetry, V.sub.b represents a total pore area
(m.sup.2/g) of the negative electrode measured at an electrode
density of 1.5 g/cc by mercury intrusion porosimetry, and 0.2 is
calculated from the equation: 1.7 g/cc-1.5 g/cc.
In another general aspect, a lithium secondary battery includes a
negative electrode including a negative electrode active material
layer, wherein the negative electrode active material layer
includes a mixed negative electrode active material including
graphite particles and low crystalline carbon-based particles, and,
in three or more points having an area of 80 .mu.m.times.20 .mu.m
optionally selected from sections of the negative electrode active
material layer, an area of the low crystalline carbon-based
particles accounts for no less than 0.5% and no more than 8% with
respect to the total area of the sections.
In the three or more points having an area of 80 .mu.m.times.20
.mu.m optionally selected from sections of the negative electrode
active material layer, the area of the low crystalline carbon-based
particles having a long diameter of 8 .mu.m or less may account for
no less than 0.5% and no more than 8% with respect to the total
area of the sections.
In the three or more points having an area of 80 .mu.m.times.20
.mu.m optionally selected from sections of the negative electrode
active material layer, a ratio of a diameter of the low crystalline
carbon-based particles to a diameter of the graphite particles may
be less than or equal to 1/2.
The diameter of the graphite particles and the diameter of the low
crystalline carbon-based particles are the highest value selected
from the long diameters of the graphite particles and the highest
value selected from the long diameters of the low crystalline
carbon-based particles, respectively, in the optionally selected
points having an area of 80 .mu.m.times.20 .mu.m.
The low crystalline carbon-based particles may have a Raman
spectrum R value (I.sub.d/I.sub.g) of 0.9 or more.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a scanning electron microscope (SEM) image obtained by
observing a section of a negative electrode prepared in Example
3.
FIG. 2 is a scanning electron microscope image obtained by
observing a section of a negative electrode prepared in Comparative
Example 2.
FIG. 3 shows a section scanning electron microscope image and a
Raman image of a certain point in a negative electrode active
material layer of a negative electrode prepared according to one
exemplary embodiment of the present invention.
FIG. 4 shows the differential scanning calorimetry (DSC) results of
measuring the negative electrode active material layer of the
negative electrode prepared according to one exemplary embodiment
of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
All terms (including technical and scientific terms) used in the
present specification may be used as a meaning which is commonly
understood by those skilled in the art to which the present
invention pertains, unless otherwise defined. Throughout the
present specification, unless explicitly described to the contrary,
"comprising" any components will be understood to imply the further
inclusion of other elements rather than the exclusion of any other
elements. Unless explicitly described to the contrary, a singular
form also includes a plural form.
One exemplary embodiment of the present invention provides a
lithium secondary battery that may be rapidly charged and
discharged because the lithium secondary battery may rapidly adsorb
and release lithium ions to/from a negative electrode, has
excellent high-temperature storage efficiency because the capacity
of the lithium secondary battery is slightly degraded even when
kept at a high temperature of 60.degree. C., and has excellent
lifespan characteristics because the capacity of the lithium
secondary battery is slightly degraded even when repeatedly charged
and discharged.
For this purpose, it is expected that the lithium secondary battery
may be chosen as a lithium secondary battery for electric vehicles,
thereby significantly advancing a time for commercialization of
electric vehicles.
Also, the lithium secondary battery may have further improved
lifespan characteristics because the lithium secondary battery has
an excellent ability to adsorb and release lithium ions, and also
shows a few side reactions with an electrolyte solution of the
negative electrode.
The lithium secondary battery according to one exemplary embodiment
of the present invention may have excellent battery output
characteristics such as charge and discharge outputs, and rapid
charging and discharging.
Further, the lithium secondary battery according to one exemplary
embodiment of the present invention may have excellent lifespan and
high-temperature storage characteristics.
Specifically, one exemplary embodiment of the present invention
provides a lithium secondary battery, which includes a negative
electrode having a negative electrode active material layer, which
includes a mixed negative electrode active material including
graphite particles and low crystalline carbon-based particles,
wherein, in three or more points having an area of 80
.mu.m.times.20 .mu.m area optionally selected from sections of the
negative electrode active material layer of the negative electrode,
an area of the low crystalline carbon-based particles accounts for
no less than 0.5% and no more than 8% with respect to the total
area of the sections.
In the lithium secondary battery according to one exemplary
embodiment of the present invention, when the area of the low
crystalline carbon-based particles in the sections of the negative
electrode active material layer satisfies this range, output
characteristics, high-temperature storage efficiency, and lifespan
characteristics of the lithium secondary battery may be improved
without any decrease in energy density by the addition of the low
crystalline carbon-based particles.
More specifically, in the three or more points having an area of 80
.mu.m.times.20 .mu.m area optionally selected from the sections of
the negative electrode active material layer, an area of the low
crystalline carbon-based particles having a long diameter of 8
.mu.m or less may account for no less than 0.5% and no more than 8%
with respect to the total area of the sections.
When the area of the low crystalline carbon-based particles
satisfies this range, output characteristics, high-temperature
storage efficiency, and lifespan characteristics of the lithium
secondary battery may also be improved without any decrease in
energy density by the addition of the low crystalline carbon-based
particles.
The low crystalline carbon-based particles may have a Raman
spectrum R value (I.sub.d/I.sub.g) of 0.9 or more.
The R value of the low crystalline carbon-based particles refers to
a ratio (I.sub.d/I.sub.g) between an intensity (I.sub.d) of a D
band and an intensity (I.sub.g) of a G band in the Raman spectrum
for the sections of the low crystalline carbon-based particles.
The intensity (I.sub.g) of the G band refers to a peak intensity in
a wavenumber domain of no less than 1,540 cm.sup.-1 and no more
than 1,620 cm.sup.-1, and the intensity (I.sub.d) of the D band
refers to a peak intensity in a wavenumber domain of no less than
1,300 cm.sup.-1 and no more than 1,420 cm.sup.-1.
Also, because the R value of the graphite particles is less than or
equal to 0.7, the graphite particles and the low crystalline
carbon-based particles may be distinguished in the Raman
analysis.
The area of the low crystalline carbon-based particles in the
sections may be measured as an area of a region having an R value
of 0.9 or more, as measured in the optionally selected points
having an area of 80 .mu.m.times.20 .mu.m by the Raman analysis as
described above.
The area of the low crystalline carbon-based particles having a
long diameter of 8 .mu.m or less may be measured as an area of the
region having a long diameter of 8 .mu.m or less in the region
having an R value of 0.9 or more, as measured in the optionally
selected points having an area of 80 .mu.m.times.20 .mu.m by the
Raman analysis as described above.
Also, in this specification, the term "long diameter" refers to the
largest length in a length of a line connecting two points on an
edge of a closed curve-shaped particle section, as determined for
the sections of the negative electrode active material layer.
Specifically, the low crystalline carbon-based particles may be
confirmed and the area of the low crystalline carbon-based
particles may be measured from the data obtained at the points
having an R value of 0.9 or more, for example, by cutting a
negative electrode active material layer of a negative electrode
specimen using an ion miller, processing sections of the negative
electrode active material layer, and subjecting the sections to
Raman analysis at longitudinal and vertical intervals of 200 nm
from any points having an area of 80 .mu.m.times.20 .mu.m using a
Raman analyzer (Nanophoton, RAMANforce) to obtain Raman data for
the electrode sections. In this case, the analysis may be performed
under conditions of a laser wavelength of 532.06 nm, a laser output
of 11.87 mW, and a laser exposure time of 20 seconds using a Raman
spectrometer.
In the three or more points having an area of 80 .mu.m.times.20
.mu.m optionally selected from the sections of the negative
electrode active material layer of the negative electrode in the
lithium secondary battery according to one exemplary embodiment of
the present invention, a ratio of a diameter of the low crystalline
carbon-based particles to a diameter of the graphite particles
(i.e., a diameter of low crystalline carbon-based particles/a
diameter of graphite particles) may be less than or equal to
1/2.
In the lithium secondary battery according to one exemplary
embodiment of the present invention, when the ratio between the
diameters of the low crystalline carbon-based particles and the
graphite particles in the sections of the negative electrode active
material layer satisfies this range, output characteristics,
high-temperature storage efficiency, and lifespan characteristics
of the lithium secondary battery may be improved without causing
any damage to the graphite particles and any decrease in energy
density by the addition of the low crystalline carbon-based
particles.
Here, the diameter of the graphite particles and the diameter of
the low crystalline carbon-based particles are the highest value
selected from the long diameters of the graphite particles and the
highest value selected from the long diameters of the low
crystalline carbon-based particles, respectively, in the optionally
selected points having an area of 80 .mu.m.times.20 .mu.m.
Specifically, the graphite particles may be confirmed with the
naked eye from a scanning electron microscope image at any points
having the area, and the diameter of the graphite particles may be
determined by measuring the highest value selected from the long
diameters of the graphite particles thus confirmed.
The low crystalline carbon-based particles may be confirmed from
the points having an R value (a ratio I.sub.d/I.sub.g of an
intensity (I.sub.d) of a D band and an intensity (I.sub.g) of a G
band in the measured Raman spectrum) of 0.9 or more based on the
Raman analysis results of the corresponding specimen as described
above, and the diameter of the low crystalline carbon-based
particles may be determined by measuring the highest value selected
from the long diameters of the low crystalline carbon-based
particles thus confirmed.
The ratio of the diameter of the low crystalline carbon-based
particles to the diameter of the graphite particles may be
illustratively and more specifically less than or equal to 1/4. In
this case, the lower limit of the ratio may be less than or equal
to 1/10.
The lithium secondary battery according to one exemplary embodiment
of the present invention may include a negative electrode having a
negative electrode active material layer, which includes a mixed
negative electrode active material including graphite particles and
low crystalline carbon-based particles, wherein the negative
electrode active material layer of the negative electrode may have
an apex of an exothermic peak in a temperature range of no less
than 370.degree. C. and no more than 390.degree. C., as measured by
differential scanning calorimetry (DSC).
More specifically, the negative electrode active material layer may
have an apex of an exothermic peak in a temperature range of no
less than 375.degree. C. and no more than 390.degree. C., or no
less than 375.degree. C. and no more than 385.degree. C.
Here, the apex of the exothermic peak also refers to the maximum
point of a peak in the corresponding temperature range.
When these physical properties are realized, output
characteristics, high-temperature storage efficiency, and lifespan
characteristics of the lithium secondary battery may be improved
without any decrease in energy density by the addition of the low
crystalline carbon-based particles. More specifically, the negative
electrode active material layer may have an apex of an exothermic
peak in a temperature range of no less than 450.degree. C. and no
more than 510.degree. C. Further more specifically, the negative
electrode active material layer may have an apex of an exothermic
peak in a temperature range of no less than 460.degree. C. and no
more than 500.degree. C.
Here, the differential scanning calorimetry measurement may, for
example, be performed by heating 6 mg of a specimen of the negative
electrode active material layer to 600.degree. C. at a rate of
5.degree. C./min while feeding air to the specimen at a rate of 50
mL/min using differential scanning calorimetry (851e Model from
Mettler Toledo Ltd.).
Also, a heating value of the exothermic peak having an apex at a
temperature of no less than 370.degree. C. and no more than
390.degree. C. may be no less than 50 J/g and no more than 200 J/g.
When the heating value of the exothermic peak satisfies this range,
output characteristics, high-temperature storage efficiency, and
lifespan characteristics of the lithium secondary battery may be
improved without any decrease in energy density by the addition of
the low crystalline carbon-based particles.
In addition, a heating value of the exothermic peak having an apex
at a temperature of no less than 450.degree. C. and no more than
510.degree. C. may be no less than 20 J/g and no more than 60 J/g.
When the heating value of the exothermic peak satisfies this range,
output characteristics, high-temperature storage efficiency, and
lifespan characteristics of the lithium secondary battery may be
improved without any decrease in energy density by the addition of
the low crystalline carbon-based particles.
The low crystalline carbon-based particles may have a Raman
spectrum R value (I.sub.d/I.sub.g) of 0.9 or more.
Meanwhile, upon thermogravimetric analysis (TGA) of the negative
electrode in the lithium secondary battery according to one
exemplary embodiment of the present invention, the weight loss of
the negative electrode at a temperature to 300.degree. C. may be no
less than 1% by weight and no more than 2% by weight, the weight
loss at a temperature from 300.degree. C. to 450.degree. C. may be
no less than 1.5% by weight and no more than 4% by weight, and the
weight loss at a temperature from 300.degree. C. to 650.degree. C.
may be no less than 35% by weight and no more than 75% by weight,
but the present invention is not particularly limited thereto.
Also, in a curve graph obtained by differentiating a weight loss
curve for thermogravimetric measurements, peaks may exist in a
temperature range of no less than 300.degree. C. and no more than
400.degree. C., and further peaks may exist in a temperature range
of no less than 400.degree. C. and no more than 500.degree. C., but
the present invention is not particularly limited thereto.
When the weight loss in the thermogravimetric analysis and a peak
range on a differential graph for the weight loss curve are
satisfied, output characteristics, high-temperature storage
efficiency, and lifespan characteristics of the lithium secondary
battery may be improved without any decrease in energy density by
the addition of the low crystalline carbon-based particles.
In the lithium secondary battery according to one exemplary
embodiment of the present invention, the low crystalline
carbon-based particles may be capable of adsorbing and releasing Li
ions.
Also, the low crystalline carbon-based particles may have a
discharge capacity of 240 mAh/g or more for the second cycle when
the negative electrode active material is used as the low
crystalline carbon-based particles and a lithium metal is used as a
counter electrode.
Here, the discharge capacity may be a discharge capacity determined
under charging/discharging conditions at which, after charging is
performed with a constant current at a rate of 0.1C, the charging
is stopped when a current flows at a rate of 0.01C in a constant
voltage mode until a constant voltage reaches 0.005 V, and
discharging to 1.5V is performed with a constant current at a rate
of 0.1C with a rest interval of 10 minutes.
Also, the low crystalline carbon-based particles may be low
crystalline carbon-based particles such as hard carbon particles,
soft carbon particles, and the like. More specifically, the low
crystalline carbon-based particles may be hard carbon
particles.
Hereinafter, when it is assumed that the low crystalline
carbon-based particles are hard carbon particles, for example, low
crystalline carbon-based particles of the present invention and a
negative electrode including the same will be described in further
detail. However, it should be understood that the low crystalline
carbon-based particles of the present invention are not intended to
be particularly limited to the hard carbon particles.
In the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, the
hard carbon particles may have a D.sub.v50 of 4 .mu.m or less and a
D.sub.n50 of 1 .mu.m or less.
The D.sub.v50 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 50% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n50 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 50% in
measuring a particle size distribution using a laser scattering
method.
When the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention
includes finely divided hard carbon particles, which satisfy the
aforementioned particle size distribution, as the negative
electrode active material, the finely divided hard carbon particles
may be disposed in pores between graphite particles serving as a
main active material. Therefore, because a volume of the negative
electrode does not increase, a decrease in energy density may not
be caused.
At the same time, because a lithium (Li) diffusion path may be
reduced, and a pathway of electrons and ions disposed in the pores
between the graphite particles may be enlarged due to the inherent
characteristics and a finely divided form of hard carbon, it is
possible to realize excellent output characteristics,
high-temperature storage efficiency, and lifespan characteristics
of the lithium secondary battery including the hard carbon.
Also, hard carbon has a sharp end. In this case, because the finely
divided hard carbon particles are disposed in the pores between the
graphite particles and have a small size of 1/2 or less with
respect to commercially available hard carbon, it is possible to
disperse stress and prevent damage to graphite particles in a
pressing stage during electrode preparation.
Specifically, the D.sub.v50 and D.sub.n50 of the hard carbon
particles measured by a laser scattering method are less than or
equal to 4 .mu.m and less than or equal to 1 .mu.m, respectively,
and the count of particles that are generally finely divided and
have a particle diameter of 1 .mu.m or less may account for 50% or
more so that the hard carbon particles can be more easily disposed
in the pores between the graphite particles, thereby realizing the
aforementioned effects.
More specifically and illustratively, the D.sub.v50 of the hard
carbon particles may be less than or equal to 3 .mu.m.
Also, when the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention
includes finely divided hard carbon particles having a small
average particle diameter, the finely divided hard carbon particles
may be disposed in the pores between the graphite particles, and
thus the number of the finely divided hard carbon particles may
increase with respect to the weight thereof when the finely divided
hard carbon particles are added at the same weight. Therefore,
excellent output characteristics, high-temperature storage
efficiency (a high-temperature storage capacity retention rate),
and lifespan characteristics of the lithium secondary battery may
be realized without any decrease in energy density even when the
finely divided hard carbon particles are added at a low
content.
That is, a mixture obtained by mixing at least 10% by weight of the
hard carbon particles of the present invention having physical
properties, which do not satisfy the particle size distribution
with the sacrifice of the energy density, with graphite particles
has been used in the art to improve the output performance.
However, when the negative electrode active material included in
the negative electrode of the lithium secondary battery according
to one exemplary embodiment of the present invention is mixed with
a small amount of the hard carbon particles satisfying a certain
particle size distribution, excellent lifespan characteristics,
high-temperature storage efficiency, and lifespan characteristics
of the lithium secondary battery may be realized without any
decrease in energy density.
Here, the D.sub.v50 and D.sub.n50 of the prepared hard carbon
particles may, for example, be measured using Mastersizer 3000
(Malvern Instruments Ltd.) after a specimen of the hard carbon
particles is taken according to the KS A ISO 13320-1 standard.
Specifically, a volume density and a number density of the hard
carbon particles may be measured after the hard carbon particles
are dispersed using ethanol as a solvent, and using an ultrasonic
disperser, when necessary.
In the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, the
mixed negative electrode active material may include 5% by weight
or less of the hard carbon particles, based on a total of 100% by
weight of the mixed negative electrode active material.
That is, in the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, the
mixed negative electrode active material may include 95% by weight
or more of the graphite particles and 5% by weight or less of the
hard carbon particles, based on a total of 100% by weight of the
mixed negative electrode active material.
That is, even when the negative electrode active material according
to one exemplary embodiment of the present invention includes a
small amount of the finely divided hard carbon particles satisfying
this particle size range, it is possible to realize output
characteristics, high-temperature storage efficiency, and lifespan
characteristics of the lithium secondary battery.
That is, when an amount of the mixed hard carbon particles is less
than or equal to 5% by weight, based on a total of 100% by weight
of the graphite particles and the hard carbon particles, output
characteristics, high-temperature storage efficiency, and lifespan
characteristics of the lithium secondary battery may be improved
without any decrease in energy density.
Also, because only a small amount of the finely divided hard carbon
particles is mixed with respect to the graphite particles, there is
no difficulty in preparing a slurry due to an increase in specific
surface area of the active material.
More specifically, an amount of the mixed hard carbon particles may
be no less than 1% by weight and no more than 5% by weight, no less
than 3% by weight and no more than 5% by weight, no less than 2% by
weight and no more than 5% by weight, no less than 2% by weight and
no more than 4% by weight, or no less than 2% by weight and no more
than 3% by weight. In this case, the graphite particles may be
included as the balance, but the present invention is not
particularly limited thereto.
Also, according to one exemplary embodiment of the present
invention, the graphite particles may be natural graphite or
artificial graphite, but the present invention is not particularly
limited thereto. The graphite particles may have a shape such as a
spherical shape, a planar shape, or the like without
limitation.
According to one exemplary embodiment of the present invention, the
average particle diameter of the graphite particles may also be no
less than 6 .mu.m and no more than 20 .mu.m. More specifically, the
average particle diameter of the graphite particles may be no less
than 8 .mu.m and no more than 17 .mu.m. Within this range, it is
desirable that the hard carbon particles according to one exemplary
embodiment of the present invention may be disposed in the pores
between the graphite particles without causing any damage to the
graphite particles.
In the negative electrode active material included in the negative
electrode of the lithium secondary battery according to one
exemplary embodiment of the present invention, the D.sub.n50 of the
hard carbon particles may be more specifically less than or equal
to 0.6 .mu.m.
Also, the D.sub.v50 may be greater than or equal to 1 .mu.m, and
the D.sub.n50 may be greater than or equal to 0.3 .mu.m, but the
present invention is not limited thereto.
In the negative electrode active material included in the negative
electrode of the lithium secondary battery according to one
exemplary embodiment of the present invention, the D.sub.v90 and
D.sub.n90 of the hard carbon particles may be no more than 8 .mu.m
and no more than 2.7 .mu.m, respectively. When the D.sub.n90 of the
hard carbon particles is less than or equal to 2.7 .mu.m, many
number fractions of the hard carbon particles may be disposed in
the pores between the graphite particles. Therefore, output
characteristics, high-temperature storage efficiency, and lifespan
characteristics of the lithium secondary battery may be improved
without any decrease in energy density of an electrode.
The D.sub.v90 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 90% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n90 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 90% in
measuring a particle size distribution using a laser scattering
method.
As can be seen from Examples as will be described below, when the
D.sub.v90 and D.sub.n90 of the hard carbon particles mixed with the
negative electrode active material included in the negative
electrode of the lithium secondary battery according to one
exemplary embodiment of the present invention satisfy these ranges,
excellent output characteristics, high-temperature storage
efficiency of the lithium secondary battery may be realized.
More specifically and illustratively, the D.sub.v90 and D.sub.n90
of the hard carbon particles may be less than or equal to 6 .mu.m
and less than or equal to 2 .mu.m, respectively.
Also, the D.sub.v90 may be greater than or equal to 4 .mu.m, and
the D.sub.n90 may be greater than or equal to 1.2 .mu.m, but the
present invention is not limited thereto.
In the negative electrode active material included in the negative
electrode of the lithium secondary battery according to one
exemplary embodiment of the present invention, the D.sub.v10 and
D.sub.n10 of the hard carbon particles may be less than or equal to
2 .mu.m and less than or equal to 0.5 .mu.m, respectively.
The D.sub.v10 represents a particle diameter when a cumulative
volume from a small particle diameter accounts for 10% in measuring
a particle size distribution using a laser scattering method, and
the D.sub.n10 represents a particle diameter when a cumulative
particle number from a small particle diameter accounts for 10% in
measuring a particle size distribution using a laser scattering
method.
As can be seen from Examples as will be described below, when the
D.sub.v10 and D.sub.n10 of the hard carbon particles mixed with the
negative electrode active material included in the negative
electrode of the lithium secondary battery according to one
exemplary embodiment of the present invention satisfy these ranges,
excellent output characteristics, high-temperature storage
efficiency of the lithium secondary battery may be realized.
More specifically and illustratively, the D.sub.v10 and D.sub.n10
of the hard carbon particles may be less than or equal to 1.5 .mu.m
and less than or equal to 0.3 .mu.m, respectively.
Also, the D.sub.v10 may be greater than or equal to 0.6 .mu.m, and
the D.sub.n10 may be greater than or equal to 0.2 .mu.m, but the
present invention is not limited thereto.
In the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, a
BET specific surface area of the negative electrode active material
layer may be no less than 2 m.sup.2/g and no more than 3.5
m.sup.2/g.
The BET specific surface area may, for example, be measured in a
pressure range (P/P.sub.0) of 0.05 to 0.3 by a nitrogen gas
adsorption BET method using an ASAP2020 apparatus (Micrometrics
Instrument Corp.) after a specimen is taken according to the KS A
0094 and KS L ISO 18757 standards and degassed at 300.degree. C.
for 3 hours using a preprocessor.
The specific surface area of the negative electrode is similar to
those of the negative electrodes in which hard carbon particles
available in the art are mixed at a content of 20% by weight. For
example, even when the hard carbon particles of the present
invention satisfying the particle size distribution are added at a
small amount of 5% by weight or less, the negative electrode of the
lithium secondary battery according to one exemplary embodiment of
the present invention may have a specific surface area similar to
the negative electrodes in which common hard carbon is mixed at a
content of 20% by weight.
That is, even when the hard carbon particles of the present
invention satisfying the particle size distribution are added at a
small amount, an effect of increasing a specific surface area of an
electrode may be realized and a lithium adsorption/release
capability may be improved without causing any damage to the
graphite particles.
For the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, a
total pore area in the negative electrode may be less than or equal
to 3.0 m.sup.2/g, as measured by mercury intrusion porosimetry when
the negative electrode is pressed at an electrode density of 1.6
g/cc. More specifically, the total pore area may be no less than
1.5 m.sup.2/g and no more than 2.9 m.sup.2/g, or no less than 1.5
m.sup.2/g and no more than 2.7 m.sup.2/g.
Also, the total pore area in the negative electrode may be less
than or equal to 2.4 m.sup.2/g, as measured by mercury intrusion
porosimetry when the negative electrode is pressed at an electrode
density of 1.5 g/cc. More specifically, the total pore area may be
no less than 1.2 m.sup.2/g and no more than 2.4 m.sup.2/g.
Further, the total pore area in the negative electrode may be less
than or equal to 3.0 m.sup.2/g, as measured by mercury intrusion
porosimetry when the negative electrode is pressed at an electrode
density of 1.7 g/cc. More specifically, the total pore area may be
no less than 2.0 m.sup.2/g and no more than 3.0 m.sup.2/g.
Here, the term "total pore area" may refer to a total surface area
of pores in the negative electrode, as measured by mercury
intrusion porosimetry. For example, the total pore area may be
measured under the following conditions using an AutoPore V 9600
mercury intrusion porosimeter (Mercury Porosimetry
Micromeritics).
<Specimen> Specimen Weight: 1 g.+-.0.1 g Electrode Specimen
Sampling: Number Corresponding to Weight of Specimen with Size of 1
cm.times.5 cm
<Measurement Medium> Mercury
<Measurement Conditions> Measured in Approximately 150 Points
from 0.2 Psig to 33,000 Psig Intervals upon Mercury Intrusion: 10
Seconds Mercury Contact Angle Setup: 130.degree. C.
Such a total pore area indicates that an increase in the total pore
area caused by an increase in electrode density is small with
respect to the negative electrodes prepared by mixing hard carbon
available in the art.
An increase in the total pore area during pressing means causing
damage (e.g., cracks, and the like) to the graphite particles
caused by the mixed hard carbon. In this case, side reactions with
an electrolyte solution may increase, and lifespan characteristics
and high-temperature storage efficiency may be degraded.
In the lithium secondary battery according to one exemplary
embodiment of the present invention, the electrode density of the
negative electrode when the negative electrode is pressed at an
electrode density of 1.6 g/cc is similar to this electrode density
range. Therefore, an increase in the total pore area caused by an
increase in the electrode density may be small with respect to the
negative electrodes prepared by mixing hard carbon available in the
art.
Therefore, the damage to the graphite particles serving as the main
active material caused by the mixed hard carbon may be prevented,
thereby reducing side reactions with an electrolyte solution and
improving lifespan characteristics and high-temperature storage
capacity efficiency.
Specifically, a rate of increase (A) in the total pore area of the
negative electrode of the lithium secondary battery according to
one exemplary embodiment of the present invention may be less than
or equal to 3.4. A=(V.sub.a-V.sub.b)/0.2
The rate of increase (A) in the total pore area is a rate of
increase in the total pore area with respect to an increase in the
electrode density when the electrode density of the negative
electrode changes from 1.5 g/cc to 1.7 g/cc. In the A, V.sub.a
represents a total pore area (m.sup.2/g) of the negative electrode
measured at an electrode density of 1.7 g/cc by mercury intrusion
porosimetry, V.sub.b represents a total pore area (m.sup.2/g) of
the negative electrode measured at an electrode density of 1.5 g/cc
by mercury intrusion porosimetry, and 0.2 is calculated from the
equation: 1.7 g/cc-1.5 g/cc.
That is, in the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention, an
increase in the total pore area caused by an increase in the
electrode density is small, indicating that the damage (e.g.,
cracks, and the like) to the graphite particles caused by the mixed
hard carbon during pressing is prevented.
Because the negative electrode of the lithium secondary battery
according to one exemplary embodiment of the present invention has
a rate of increase (A) in the total pore area within this range, an
increase in the total pore area of the negative electrode caused by
an increase in the electrode density may be small with respect to
the negative electrodes prepared by mixing hard carbon available in
the art.
Therefore, the damage to the graphite particles caused by the mixed
hard carbon may be prevented, thereby reducing side reactions with
an electrolyte solution and improving lifespan characteristics and
high-temperature storage capacity efficiency.
The lower limit of A may be 2.0, particularly 2.5, but the present
invention is not particularly limited thereto.
The upper limit of A may be more specifically 3.2, but the present
invention is not particularly limited thereto.
Hereinafter, other components other than the negative electrode
active material in the negative electrode will be described.
However, it should be understood that the description provided
herein is given by way of example, but is not particularly intended
to limit the present invention.
The negative electrode may be prepared by mixing a solvent, and,
when necessary, a negative electrode binder, and a conductive
material with a negative electrode active material, stirring the
resulting mixture to prepare a slurry, coating a current collector
with the slurry, compressing the current collector, and drying the
current collector to form a negative electrode active material
layer on the current collector. The negative electrode active
material is as described above, and thus a description thereof is
omitted.
Hereinafter, the current collector, the negative electrode binder,
and the conductive material will be described in further detail.
However, it should be understood that the description provided
herein is not intended to limit the present invention.
The negative electrode binder serves to readily attach negative
electrode active material particles to each other, and also serves
to readily attach a negative electrode active material to a current
collector. Water-insoluble binder, water-soluble binder, or a
combination thereof may be used as the binder.
The water-insoluble binder may include polyvinyl chloride,
carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer
including ethylene oxide, polyvinylpyrrolidone, polyurethane,
polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,
polypropylene, polyamide imide, polyimide, or a combination
thereof.
The water-soluble binder may include a styrene-butadiene rubber, an
acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium
polyacrylate, an olefin copolymer with propylene and 2 to 8 carbon
atoms, a copolymer of (meth)acrylic acid and (meth)acrylic acid
alkyl ester, or a combination thereof.
When a water-soluble binder is used as the negative electrode
binder, the binder may further include a cellulose-based compound
capable of giving viscosity. The cellulose-based compound may
include carboxymethyl cellulose, hydroxypropylmethyl cellulose,
methyl cellulose, or an alkali metal salt thereof, which may be
used alone or in combination with one or more types thereof. Na, K,
or Li may be used as the alkali metal.
The conductive material is used to give conductivity to an
electrode. For the battery thus configured, any conductive
materials may be used as long as they are materials that are
electronically conductive without causing any chemical change. For
example, conductive materials including a carbon-based material
such as natural graphite, artificial graphite, carbon black,
acetylene black, ketjen black, carbon fibers, carbon nanotubes, and
the like; a metallic material such as a metal powder, for example,
copper, nickel, aluminum, silver, and the like, metal fibers, or
the like; a conductive polymer such as a polyphenylene derivative,
or the like; or a mixture thereof may be used as the conductive
material.
In addition, one selected from the group consisting of a copper
film, a nickel film, a stainless steel film, a titanium film, a
nickel foam, a copper foam, a polymer substrate coated with a
conductive metal, or a combination thereof may be used as the
current collector.
Hereinafter, configurations of other components other than the
negative electrode in the lithium secondary battery according to
one exemplary embodiment of the present invention will be
described.
The lithium secondary battery may include the aforementioned
negative electrode according to one exemplary embodiment of the
present invention, a positive electrode, and an electrolyte, and
may further include a separator between the positive electrode and
the negative electrode.
The negative electrode is as described above, and thus the other
components of the battery will be described below. However, it
should be understood that the description provided herein is given
by way of example, but is not intended to limit the present
invention.
The positive electrode may include a current collector and a
positive electrode active material layer disposed on the current
collector. Al or Cu may be used as the current collector, but the
present invention is not limited thereto.
A compound (a lithiated intercalation compound) enabling reversible
intercalation or deintercalation of lithium ions may be used as the
positive electrode active material. Specifically, one or more of
complex oxides of lithium and metals selected from cobalt,
manganese, nickel, and a combination thereof as known in the
related art may, for example, be used as the lithium metal oxide.
In this case, certain compositions of the lithium metal oxide are
not particularly limited.
The positive electrode active material layer may further include a
positive electrode binder and a conductive material.
The binder serves to readily attach positive electrode active
material particles to each other, and also serves to readily attach
a positive electrode active material to a current collector.
Polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl
cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated
polyvinyl chloride, polyvinyl fluoride, a polymer including
ethylene oxide, polyvinylpyrrolidone, polyurethane,
polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,
polypropylene, a styrene-butadiene rubber, an acrylated
styrene-butadiene rubber, an epoxy resin, nylon, and the like may
be used as representative examples of the binder, but the present
invention is not limited thereto.
The conductive material is used to give conductivity to an
electrode. For the battery thus configured, any conductive
materials may be used as long as they are materials that are
electronically conductive without causing any chemical change. For
example, metal powders such as natural graphite, artificial
graphite, carbon black, acetylene black, ketjen black, carbon
fibers, carbon nanotubes, copper, nickel, aluminum, silver, and the
like; metal fibers, and the like may be used as the conductive
material. Also, a conductive material such as a polyphenylene
derivative, and the like may be used alone or in combination with
one or more types thereof, but the present invention is not limited
thereto.
The lithium secondary battery may be a non-aqueous electrolyte
secondary battery. In this case, a non-aqueous electrolyte may
include a non-aqueous organic solvent and a lithium salt.
The non-aqueous organic solvent may serve as a medium that enables
the movement of ions involved in an electrochemical reaction of the
battery.
Materials commonly used in the technical field of lithium secondary
batteries may be used as the non-aqueous organic solvent and the
lithium salt. In this case, certain materials are not limited
thereto.
Also, as described above, a separator may also be present between
the positive electrode and the negative electrode. Polyethylene,
polypropylene, polyvinylidene fluoride, or a multilayered film of
two or more types thereof may be used as the separator. Of course,
a mixed multilayered film such as a polyethylene/polypropylene
2-layered separator, a polyethylene/polypropylene/polyethylene
3-layered separator, a polypropylene/polyethylene/polypropylene
3-layered separator, and the like may be used herein, but the
present invention is not particularly limited thereto.
Hereinafter, preferred examples and comparative examples of the
present invention will be described. It should be understood that
the following examples are merely given as the preferred examples
of the present invention, but are not intended to limit the present
invention.
PREPARATION EXAMPLES
As summarized in the following Table 1, hard carbon particles of
Preparation Example 1, and commercially available hard carbon
particles of Preparation Example 2 were prepared as the low
crystalline carbon-based particles having a Raman spectrum R value
(I.sub.d/I.sub.g) of 0.9 or more.
TABLE-US-00001 TABLE 1 Particle Size Distribution (.mu.m) Items
Type D10 D50 D90 Preparation Volume Density 1.2 3.0 6.0 Example 1
Number Density 0.3 0.6 1.8 Preparation Volume Density 2.4 7.7 13
Example 2 Number Density 0.6 1.1 2.8
The particle size distributions of the hard carbon particles of
Preparation Examples 1 and 2 were measured using Mastersizer 3000
(Malvern Instruments Ltd.) after a specimen of the hard carbon
particles was taken according to the KS A ISO 13320-1 standard.
Specifically, a volume density and a number density of the hard
carbon particles were measured after the hard carbon particles were
dispersed using ethanol as a solvent, and using an ultrasonic
disperser, when necessary.
EXAMPLES AND COMPARATIVE EXAMPLES
1. Preparation of Negative Electrode and Coin-Type Half Cell
The hard carbon particles of Preparation Example 1 or 2 were mixed
with spherical natural graphite (a main active material) having an
average particle diameter of 11 .mu.m to prepare a negative
electrode active material. Mixing ratios (by weight) of spherical
natural graphite and hard carbon particles used in each of Examples
and Comparative Examples are summarized in Table 2.
Next, the prepared negative electrode active material, a styrene
butadiene rubber (SBR), and carboxymethyl cellulose (CMC) serving
as a thickening agent were mixed at a mass ratio of 97.3:1.5:1.2,
and the resulting mixture was then dispersed in ion-free distilled
water to prepare a composition. Thereafter, one surface of a
Cu-foil current collector was coated with the composition, and the
Cu-foil current collector was dried and rolled to form a negative
electrode active material layer having a size of 10 cm.times.10
cm.times.50 .mu.m. As a result, a negative electrode having an
electrode density of 1.60.+-.0.05 g/cm.sup.3 and a loading amount
of 8 mg/cm.sup.2 was prepared.
Lithium metal films were used as the negative electrode thus
prepared and a counter electrode. And, the negative electrode and
counter electrode were impregnated into an electrolyte solution in
which 1 M LiPF.sub.6 was dissolved in a mixed solvent of ethylene
carbonate (EC)/ethyl methyl carbonate (EMC)/diethylene carbonate
(DEC) (at a volume ratio of 25:45:30), with a separator
(polyethylene having a thickness of 25 .mu.m) interposed between
the negative electrode and the counter electrode, to prepare a
2016-type coin cell.
2. Preparation of Negative Electrode and Full Cell
A negative electrode was prepared in the same manner as in
"Preparation of Half Cell," except that a solution obtained by
mixing spherical natural graphite having an average particle
diameter of 11 .mu.m and artificial graphite having an average
particle diameter of 13 .mu.m at a weight ratio of 5:5 was used as
the main active material.
Li.sub.1.0Ni.sub.0.6Co.sub.0.2Mn.sub.0.2O.sub.2, Denka Black, and
PVDF were used as the positive electrode active material, the
conductive material, and the binder, respectively, and N-methyl
pyrrolidone was used as the solvent. In this case, the respective
components were mixed with a mass ratio composition of
46:2.5:1.5:50 to prepare a positive electrode mixture. Thereafter,
an aluminum substrate was coated with the positive electrode
mixture, dried, and pressed to prepare a positive electrode.
The prepared positive and negative electrodes were separately
notched with a proper size, and stacked, and a separator
(polyethylene having a thickness of 25 .mu.m) was interposed
between a positive electrode plate and a negative electrode plate
to constitute a battery. Thereafter, a tap region of the positive
electrode and a tap region of the negative electrode were
separately welded.
The welded positive electrode/separator/negative electrode assembly
was put into a pouch, and three sides other than a side of
electrolyte solution immersion portion (including a tapped region
in a sealing region) were sealed. An electrolyte solution was
immersed into the other region, and the other side was sealed, and
the assembly was then impregnated for 12 hours or more. The
electrolyte solution was a mixed solvent of ethylene carbonate
(EC)/ethyl methyl carbonate (EMC)/diethylene carbonate (DEC) (at a
volume ratio of 25:45:30). In this case, a solution obtained by
preparing a 1 M LiPF.sub.6 solution and adding 1% by weight of
vinylene carbonate (VC), 0.5% by weight of 1,3-propenesultone
(PRS), and 0.5% by weight of lithium bis(oxalato)borate (LiBOB) to
the solution was used as the electrolyte solution.
Next, the electrolyte solution was pre-charged for 36 minutes at a
current (2.5 A) corresponding to 0.25C. After an hour, the
electrolyte solution was degassed, aged for 24 hours, and then
subjected to formation charging and discharging (charging
conditions: CC-CV 0.2C 4.2 V 0.05C CUT-OFF, and discharging
conditions: CC 0.2C 2.5 V CUT-OFF).
Then, the standard charging and discharging were performed
(charging conditions: CC-CV 0.5 C 4.2 V 0.05C CUT-OFF, and
discharging conditions: CC 0.5C 2.5 V CUT-OFF).
TABLE-US-00002 TABLE 2 Negative Electrode Active Material
Compositions (% represents % by weight) Mixing Ratio of Mixing Hard
Carbon Main Active Ratio of Items Types Material Hard Carbon
Example 1 Preparation 99% 1% Example 1 Example 2 Preparation 98% 2%
Example 1 Example 3 Preparation 97% 3% Example 1 Example 4
Preparation 96% 4% Example 1 Example 5 Preparation 95% 5% Example 1
Comparative Preparation 97% 3% Example 1 Example 2 Comparative
Preparation 80% 20% Example 2 Example 2
FIG. 1 is a scanning electron microscope (SEM) image obtained by
observing a section of the negative electrode prepared in Example
3. It can be seen that the hard carbon particles of the present
invention satisfying the particle size distribution were disposed
on the pores between the natural graphite particles, and the
natural graphite was not damaged.
FIG. 2 is a scanning electron microscope image obtained by
observing a section of the negative electrode prepared in
Comparative Example 2. It can be seen that natural graphite was
damaged due to the large size of hard carbon.
Experimental Example 1
Certain sections of the negative electrodes prepared during
preparation of the full cells of Example 5 and Comparative Example
2 were photographed using a scanning electron microscope, and the
Raman spectra of the same sections were measured. Based on these
results, a ratio of a diameter of the hard carbon particles to a
diameter of the graphite particles in the negative electrode active
material layer, and an area of the hard carbon particles having a
long diameter of 8 .mu.m or less in the sections of the negative
electrode active material layer were measured.
Specifically, the Raman spectrum measurement was performed by
cutting the negative electrode active material layer of the
negative electrode using an ion miller, processing sections of the
negative electrode active material layer, and subjecting the
sections to Raman analysis at longitudinal and vertical intervals
of 200 nm from three optionally selected points having an area of
80 .mu.m.times.20 .mu.m using a Raman analyzer (Nanophoton,
RAMANforce). In this case, a laser wavelength, a laser output, and
a laser exposure time of a Raman spectrometer were set to 532.06
nm, 11.87 mW, and 20 seconds, respectively.
A region in which the hard carbon was present was able to be
confirmed from the points having a Raman spectrum R value (a ratio
I.sub.d/I.sub.g of an intensity (I.sub.d) of a D band and an
intensity (I.sub.g) of a G band in the measured Raman spectrum) of
0.9 or more.
FIG. 3 shows a scanning electron microscope image and a Raman image
of the negative electrode prepared in Example 5 as measured in this
way.
Next, a ratio of a diameter of the hard carbon particles to a
diameter of the graphite particles (i.e., a diameter of hard carbon
particles/a diameter of graphite particles) in the sections the
negative electrode active material layer, and an area of the hard
carbon particles having a long diameter of 8 .mu.m or less in the
sections were measured using the following methods. The ratio of a
diameter of hard carbon particles to a diameter of graphite
particles was calculated using the largest one of long diameters of
the graphite particles confirmed and measured with the naked eye
from the scanning electron microscope image as the diameter of the
graphite particles and using the largest one of long diameters of
the hard carbon particles confirmed by the Raman spectrum as the
diameter of the hard carbon particles. In the area of hard carbon
particles having a long diameter of 8 .mu.m or less in the sections
of a negative electrode active material layer, an area of a portion
of the hard carbon particles having a long diameter of 8 .mu.m or
less in a portion of the hard carbon particles having a Raman
spectrum R value of 0.9 or more was calculated from the total area
of the sections of the negative electrode active material layer in
the Raman image.
From the analysis results, in the case of the negative electrode
prepared in Example 5, the ratio of the diameter of the hard carbon
particles to the diameter of the graphite particles was less than
or equal to 1/2, and the area of the hard carbon particles having a
long diameter of 8 .mu.m or less in the sections of the negative
electrode active material layer was less than or equal to 8%.
Meanwhile, in the case of the negative electrode prepared in
Comparative Example 2, the ratio of the diameter of the hard carbon
particles to the diameter of the graphite particles was greater
than 1/2, and the area of the hard carbon particles having a long
diameter of 8 .mu.m or less in the sections of the negative
electrode active material layer exceeded 8%.
Experimental Example 2
The negative electrodes prepared during preparation of the full
cells of Examples 1 to 3 and Comparative Examples 1 and 2 were
measured by differential scanning calorimetry (851e Model from
Mettler Toledo Ltd.). The control (Ref.) represents a case in which
100% by weight of spherical natural graphite having an average
particle diameter of 11 .mu.m or less was used as the negative
electrode active material without any addition of hard carbon.
Specifically, the measurement was performed by heating 6 mg of a
specimen of the negative electrode active material layer prepared
in each of Examples to 600.degree. C. at a rate of 5.degree. C./min
while feeding air to the specimen at a rate of 50 mL/min.
The results are summarized in FIG. 4.
From the measurement results, the negative electrodes prepared in
Examples 1 to 3 showed an apex of an exothermic peak in a
temperature range of no less than 370.degree. C. and no more than
390.degree. C., and the negative electrodes of Examples 2 and 3 in
which hard carbon particles were mixed at a content of 2% by weight
or more also showed an apex of an exothermic peak in a temperature
range of no less than 450.degree. C. and no more than 510.degree.
C. On the other hand, the negative electrodes prepared in
Comparative Examples 1 and 2 did not show an apex of an exothermic
peak in a temperature range of no less than 370.degree. C. and no
more than 390.degree. C.
Also, in the case of the negative electrodes prepared in Examples 1
to 3, a heating value of the exothermic peak having an apex at a
temperature of no less than 370.degree. C. and no more than
390.degree. C. was no less than 50 J/g and no more than 200 J/g,
and a heating value of the exothermic peak having an apex at a
temperature of no less than 450.degree. C. and no more than
510.degree. C. was no less than 20 J/g and no more than 60 J/g.
On the other hand, the negative electrode prepared in Comparative
Example 2 did not satisfy this peak heating value.
Experimental Example 3
Negative electrodes were prepared according to the following
measurement methods in the same manner as in Examples 3 and 5 and
Comparative Examples 1 and 2, except that the varying electrode
densities were used. Thereafter, the total pore areas of the
prepared negative electrodes were measured by mercury intrusion
porosimetry. The results are summarized in Table 3.
<Measurement Method>
Measuring Machine: using AutoPore V 9600 mercury intrusion
porosimeter (Mercury Porosimetry Micromeritics).
<Specimen> Specimen Weight: 1 g.+-.0.1 g Electrode Specimen
Sampling: Number Corresponding to Weight of Specimen with Size of 1
cm.times.5 cm
<Measurement Medium> Mercury
<Measurement Conditions> Measured in Approximately 150 Points
from 0.2 Psig to 33,000 Psig Intervals upon Mercury Intrusion: 10
Seconds Mercury Contact Angle Setup: 130.degree. C.
TABLE-US-00003 TABLE 3 Rate of Total Pore Area (m.sup.2/g) Increase
(A) Before 1.5 1.6 1.7 in Total Items Pressing g/cc g/cc g/cc Pore
Area Example 3 1.203 2.204 2.651 2.748 2.72 Example 5 1.34 2.22
2.68 2.854 3.17 Comparative 1.41 2.727 2.935 3.422 3.48 Example 1
Comparative 1.62 2.932 3.642 4.2 6.34 Example 2
A=(V.sub.a-V.sub.b)/0.2 (wherein V.sub.a represents a total pore
area (m.sup.2/g) of the negative electrode measured at an electrode
density of 1.7 g/cc by mercury intrusion porosimetry, V.sub.b
represents a total pore area (m.sup.2/g) of the negative electrode
measured at an electrode density of 1.5 g/cc by mercury intrusion
porosimetry, and 0.2 is calculated from the equation: 1.7 g/cc-1.5
g/cc).
It was confirmed that the negative electrodes prepared in Examples
3 and 5 had a small total pore area at the same electrode density,
compared to the negative electrodes of Comparative Examples, and an
increase in the total pore area caused by an increase in electrode
density was small. From these results, it can be seen that the
damage to the graphite particles serving as the main active
material caused by the mixed hard carbon was prevented, indicating
that side reactions with an electrolyte solution were reduced and
lifespan characteristics and a high-temperature storage capacity
retention rate were improved.
Experimental Example 4
BET specific surface areas of the negative electrodes prepared in
Examples 1, 2, 3, and 5 and Comparative Example 2 were measured.
The results are summarized in Table 4.
The BET specific surface area was measured, as follows.
A specimen was taken according to the KS A 0094 and KS L ISO 18757
standards, and degassed at 300.degree. C. for 3 hours using a
preprocessor, and a specific surface area of the specimen was
measured in a pressure range (P/P.sub.0) of 0.05 to 0.3 by a
nitrogen gas adsorption BET method using an ASAP2020 apparatus
(Micrometrics Instrument Corp.).
TABLE-US-00004 TABLE 4 BET Specific Surface Area (m.sup.2/g) of
Negative Electrode Active Material Layer Example 1 2.877 Example 2
2.954 Example 3 3.031 Example 5 3.185 Comparative 2.98 Example
2
As listed in Table 4, even when the hard carbon particles of
Preparation Example 1 satisfying the particle size distribution
were added at a small amount of 5% by weight or less, the negative
electrodes had similar specific surface areas, compared to when 20%
by weight of common hard carbon was mixed. That is, it was
confirmed that, even when the hard carbon particles of the present
invention satisfying the particle size distribution were added at a
small amount, an effect of increasing a specific surface area of
the electrode was realized and a lithium adsorption/release
capability was improved without causing any damage to the natural
graphite.
Experimental Example 5
Capacity retention rates of the half cells prepared in Examples 1
and 3 and Comparative Example 1 according to the constant rate
thereof were measured. The results are summarized in the following
Table 5.
In Table 5, the control (Ref.) represents a case in which 100% by
weight of spherical natural graphite having an average particle
diameter of 11 .mu.m or less was used as the negative electrode
active material without any addition of hard carbon.
TABLE-US-00005 TABLE 5 Discharge Capacity (mAh/g) According to
C-Rate Items 0.1 C 0.2 C 1 C 2 C 3 C 4 C 5 C Example 1 355.8 354.7
346.7 258.9 201.7 171.5 -- Example 3 362.3 361.5 358.7 315.2 283.6
257 210.9 Comparative 355.2 354.3 329.4 243.5 -- -- -- Example 1
Ref. 354 353.3 327.3 237.2 -- -- --
As listed in Table 5, it was confirmed that the capacities were not
reduced in the case of the half cells of Examples 1 and 3 in which
the hard carbon particles of Preparation Example 1 satisfying the
particle size distribution were mixed.
Also, the half cell of Comparative Example 1 in which 3% by weight
of the hard carbon particles of Preparation Example 2 were mixed
was not able to be charged and discharged at a high rate.
On the other hand, it was confirmed that the half cells of Examples
1 and 3 were chargeable and dischargeable at a rate of 3C or more,
and thus were chargeable and dischargeable at a high temperature.
The half cell of Example 3 showed a relatively high capacity
retention rate even at a high rate of 5C.
Experimental Example 6
The full cells prepared in Examples 3 and 5 and Comparative Example
1 were kept for 12 weeks in the air at 60.degree. C. under a full
state of charge (SoC) of 100%, and taken out every week to perform
a residual discharge. Thereafter, the full cells were charged and
discharged to measure capacities and direct current internal
resistances (DC-IRs). Then, the high-temperature storage
performance was evaluated by comparing the capacity retention rates
and the rates of increase in the DC-IR with those before being kept
at 60.degree. C. The results are summarized in Tables 6 and 7.
The charging/discharging conditions used to determine the
capacities before and after being kept were as follows.
At 25.degree. C., charging conditions: CC-CV 0.5 C 4.2 V 0.05C
CUT-OFF, and discharging conditions: CC 0.5C 2.5 V CUT-OFF.
TABLE-US-00006 TABLE 6 Initial Capacity Retention Rate Capacity
(Ah) (%) After Being Kept Example 3 20.4 80.8 Example 5 20.1 79
Comparative 20.1 63.4 Example 1
As listed in Table 6, the full cells of Examples 3 and 5 had no
reduced capacity and showed excellent high-temperature storage
efficiency.
TABLE-US-00007 TABLE 7 Rate of Increase (%) in DC-IR at SOC of 50%
After Being Kept at 60.degree. C. for 12 Weeks Example 3 10.7
Example 5 11.0 Comparative 46.1 Example 1
As listed in Table 7, it can be seen that, even when the full cells
of Examples 3 and 5 were kept at 60.degree. C. for 12 weeks, the
rates of increase in DC-IRs of the full cells were lower than that
of Comparative Example 1.
The discharge DC-IR was measured based on the discharge capacity
measured after a full cell was kept at a high temperature, followed
by finishing capacity measurements. The measurement method was as
follows.
Discharge direct current internal resistance (DC-IR) measurement:
The discharge DC-IR may be calculated from the formula:
(V.sub.0-V.sub.1)/I.sub.dis, and a full cell is charged to a state
of charge (SOC) of 50% at room temperature and a rate of 0.5C, and
rest for 30 minutes. In this case, a voltage (V.sub.0) is measured.
A voltage (V.sub.1) when a full cell is discharged with a current
(I.sub.dis) of 1C for 10 seconds is measured.
Experimental Example 7
The full cells prepared in Examples 3 and 5 and Comparative Example
1 were repeatedly charged and discharged at 25.degree. C. under the
following condition for 600 cycles, and the capacity retention
rates with respect to the initial capacities were calculated. The
results are summarized in Table 8.
The full cell was charged with a current at a rate of 0.5C by
applying a constant current to the full cell until a battery
voltage of the full cell reached 4.2 V, and then charged by
maintaining the constant voltage when the battery voltage reached
4.2 V and cutting off the current when the current density reached
a rate of 0.05C. Then, the full cell was discharged at a constant
current at a rate of 0.5C until the voltage reached 2.5V. This
cycle was repeated 600 times.
TABLE-US-00008 TABLE 8 Initial Capacity Retention Rate Capacity
(Ah) (%) after 600 Cycles Example 3 20.4 93 Example 5 20.1 92.2
Comparative 20.1 90.7 Example 1
As listed in Table 8, it can be seen that the room-temperature
lifespan characteristics of the full cells of Examples 3 and 5 were
very excellent, and were improved compared to that of Comparative
Example 1.
The lithium secondary battery according to one exemplary embodiment
of the present invention can be rapidly charged and discharged
because lithium ions can be rapidly adsorbed and released to/from
the negative electrode, may have excellent high-temperature storage
efficiency because the capacity of the lithium secondary battery is
slightly degraded even when kept at a high temperature of
60.degree. C., and may have excellent lifespan characteristics
because the capacity of the lithium secondary battery is slightly
degraded even when repeatedly charged and discharged. Therefore, it
is expected that the lithium secondary battery of the present
invention can be chosen as a lithium secondary battery for electric
vehicles, thereby significantly advancing a time for
commercialization of electric vehicles.
Also, the lithium secondary battery according to one exemplary
embodiment of the present invention can have further improved
lifespan characteristics because the lithium secondary battery has
an excellent ability to adsorb and release lithium ions, and also
shows a few side reactions with an electrolyte solution of the
negative electrode.
The lithium secondary battery according to one exemplary embodiment
of the present invention can have excellent battery output
characteristics such as charge and discharge outputs, and rapid
charging and discharging.
Further, the lithium secondary battery according to one exemplary
embodiment of the present invention can have excellent lifespan and
high-temperature storage characteristics.
* * * * *